BlackRock, Prudential and other investment firms have sued US Bank in New York state court, alleging it ignored “pervasive and systemic” issues in the underlying loan pools of 21 residential mortgage-backed securities trusts it administers, which were secured by more than $18.3 billion at the time of securitization.

The proposed class action filed Monday alleges the bank — described as the nation’s largest corporate trustee, with nearly one-third of all structured financial trust business — has a fiduciary duty and a contractual obligation to enforce their…